Finely dice the fish into small pieces, then coarsely chop with a sharp knife. Refrigerate while preparing the seeds.
In a dry pan over low heat, toast the sunflower, flax, and sesame seeds for 5 minutes until fragrant. Let cool completely.
Gently mix the cooled seeds into the chopped fish. Season with fleur de sel and freshly ground pepper, then drizzle with olive oil. Cover and chill in the fridge until serving.
To plate, divide the tartare among chilled plates. Top with fresh green radish sprouts and garnish with a lemon slice for a burst of citrus.
